The present invention pertains to an elastomeric tie pad for a railroad track.
When railcars travel over railroad track, they are often subjected to an undesirable amount of vibration and periodic impacts that tend to dislodge cargo, damage railroad ties and railcar structures, such as wheels, degrade railroad track, and/or annoy passengers. Accordingly, much effort has been expended to design railroad tracks in such a way that minimizes these vibrations and impacts.
Railroad track typically include two parallel metal rails mounted on a plurality of transverse railroad ties, typically made of plastic, concrete, wood, or a combination thereof. The ties, in turn, are usually supported by ballast that typically comprises rock or other similar material and is laid over subgrade or other type of underlayment. In the case of “open track”, the subgrade is simply the ground, while in the case of track laid over a bridge, tunnel, or other structure, the subgrade may be concrete, wood, or other such material. In addition, it is often desirable to include an impermeable layer of subballast between the ballast and the subgrade, typically comprising compacted fine gravel.
Excessive rail car vibration can result from too little track deflection as a railcar moves over the track. Though metal rails and concrete ties will deflect somewhat under the weight of a passing rail car, the amount of deflection each contributes to the total track deflection needed for a smooth ride is relatively insignificant. Of the materials that comprise the railroad track (i.e., the rails, the ties, and the ballast), most of the deflection is provided by the ballast. Open ground can also contribute a relatively significant amount of deflection under the weight of a passing rail car. The amount of open ground deflection varies significantly depending upon the type of terrain.
The total deflection of track laid over open ground is usually sufficient to provide an adequately smooth ride. In instances where this is not the case, such as where the ground is particularly rocky, additional ballast may be provided, or wood ties may be used, which deflect more than concrete ties.
Railroad track must often be laid over structures such as tunnels, bridges, and the like that have significantly less deflection than open ground. Further, tunnels often have insufficient clearance to include an appropriate amount of ballast. Thus, a rail car that travels over or through such structures will be subjected to undesirable vibrations due to the loss of the deflection otherwise provided by the ballast and/or the open ground.
One prior art suggestion to reduce rail car vibrations in tunnels, or further reduce rail car vibrations over open ground, is to include soft elastomeric material beneath either the rails or the ties. For example, Sonneville, U.S. Pat. No. 3,289,941 suggests that a sheath of gas-injected elastomeric material beneath concrete ties in a tunnel can increase track deflection, even where the tunnel does not permit ballast.
One problem encountered with these solutions is that, even where the deflection of the track on a structure such as a tunnel or a bridge is sufficient to dampen vibrations, a rail car traveling over a bridge or tunnel may nonetheless receive a significant transition impact or shock. This transition impact results not from the steady vibrations caused by insufficient cushioning over the length of the bridge or tunnel, but instead from the boundary between the bridge, tunnel, or other structure and its adjacent approach. Further, the resulting transition impact may be transmitted along the length of a train when each rail car in the train passes over the boundary. An additional problem with existing solutions to reduce rail car vibrations in tunnels is that the soft material used for cushioning wears significantly after repeated deflections, either hardening to the point where vibration once again becomes problematical, or failing altogether.
What is desired, therefore, is an improved system for reducing vibrations and/or periodic impacts encountered as a railroad car travels over transitions between open track and structures such as bridges or tunnels.